Generally, in mobile transmission systems, interference from other users, i.e., multi-user interference is a significant factor to limit the system capacity. Thus, for increasing the system capacity, an adaptive antenna technique is considered effective since it suppresses a received signal in a particular direction (interfering wave direction) during reception, and avoids interference in unnecessary directions during transmission.
FIG. 1 illustrates a prior art example of a transmitting/receiving apparatus using such an adaptive antenna (adaptive antenna transmitting/receiving apparatus). As illustrated in FIG. 1, this conventional adaptive antenna transmitting/receiving apparatus comprises N antenna elements 1011–101N. N multipliers 1021–102N, adder 103, reception antenna weight generating circuit 104, antenna weight converting circuit 105, and N multipliers 1061–106N.
Multipliers 1021–102N perform weighted multiplications by multiplying received signals from antenna elements 1011–101N by reception antenna weights W=(w1, w2, . . . , wN) generated by reception antenna weight generating circuit 104, respectively. Adder 103 adds together the received signals of the respective antennas weighted by multipliers 1021–102N to deliver as single received data.
Reception antenna weight generating circuit 104 is applied with the received data delivered from adder 103, and generates weighting coefficients which permit the received data to have an optimal value as reception antenna weights W.
The operation of reception antenna weight generating circuit 104 is generally implemented by MMSE (minimum Mean Squared Error) control which adaptively updates weighting coefficients to minimize a mean squared error of weighted and added received signals and a reference signal. Known algorithms for realizing MMSE include LMS (Least Mean Square) and RLS (Recursive Least Square) algorithms, and the like. In the present invention, no limitations are particularly imposed to the algorithm used in reception antenna weight generating circuit 104.
Reception antenna weights W=(w1, w2, . . . , wN) generated by reception antenna weight generating circuit 104 are used for weighting received signals in multipliers 1021–102N, and are also applied to antenna weight converting circuit 105.
Antenna weight converting circuit 105 converts reception antenna weights W generated by reception antenna weight generating circuit 104 to transmission antenna weights W′=(w′1, w′2, . . . , w′N). This conversion is performed for correcting an amplitude/phase deviation among branches of an RF transmitting circuit, or for correcting a frequency difference of transmission/reception when transmission/reception are different in frequency as an FDD (Frequency Division Duplex) system, and is basically performed with the intention to generate transmission weights W′ for forming a similar pattern to a directivity pattern formed in the reception.
Multipliers 1061–106N multiply data to be transmitted by transmission antenna weights W′ delivered from antenna weight converting circuit 105, respectively.
In the conventional adaptive antenna transmitting/receiving apparatus illustrated in FIG. 1, a plurality of N antenna elements 1011–101N are arranged in an array, and weighting coefficients of respective antenna elements 1011–101N are adaptively controlled through signal processing to conduct a control for increasing an antenna gain (directivity) in a particular direction. On the other hand, in transmission, the reception antenna weights are calibrated to generate transmission antenna weights which are multiplied by signals to be transmitted of respective users, such that the resulting signal are transmitted to have the directivity in a desired wave user direction and to reduce interference given to other user directions. An adaptive antenna transmitting/receiving apparatus in such a configuration is described in literature [1] listed below.
Literature [2] listed below shows a system for controlling reception antenna weights so as to minimize a mean squared error after RAKE combination generated using a despread pilot symbol and a determination information data symbol as reference signals, for example, as a method of calculating optimal reception weights.
Also, literature [3] listed below describes a method of using transmission antenna weights which are generated based on antenna weights generated as described above, as a method of generating transmission antenna weights.
[1] NTT DoCoMo Technical Journal, Vol. 8, No. 1 (April 2000);
[2] S. Tanaka, M. Sawahashi and F. Adachi, “Pilot symbol-assisted decision-directed coherent adaptive array diversity for DS-CDMA mobile radio reverse link”, IEICE Trans. Fundamentals, Vol. E80-A, pp. 2445–2454, (December, 1997); and
[3] Tanaka, Harada, Ihara, Sawahashi and Adachi, “Outdoor experiment characteristics of adaptive antenna array diversity reception in W-CDMA, Technical Report RCS99–127, pp. 45–50 (October 1999).
In the conventional adaptive antenna transmitting/receiving apparatus illustrated in FIG. 1, the transmission antenna weights are generated based on the reception antenna weights generated for reception. This is intended to apply the directivity pattern formed during reception, as it is, to transmission. However, if the apparatus fails to generate optimal antenna weights, for example:
(1) when the generation of reception antenna weights is in course of convergence, so that optimal weights have not been formed;
(2) when the reception line quality is so bad that the generation of optimal reception antenna weights encounters difficulties;
(3) when the reception line is instantaneously interrupted due to shadowing or the like;
(4) when a failure in the apparatus or the like results in defective reception, and the like,
the transmission antenna weights are not either optimized. For this reason, it is contemplated that not only the directivity is not formed for a desired station, but also larger interference is given to other users by contraries, resulting in a significant degradation of the transmission characteristic as well as the reception characteristic.
Conventional techniques for controlling transmission antenna weights to have optimal values are described in JP-2000-22611-A, JP-2001-217759-A, and the like.
JP-2000-22611-A describes an adaptive antenna transmitting/receiving apparatus which comprises an incoming direction estimating circuit, and estimates an incoming direction by the incoming direction estimating circuit to correct transmission antenna weights. However, this conventional adaptive antenna transmitting/receiving apparatus cannot estimate an incoming direction if the reception line quality is degraded, resulting in the inability to control optimal transmission weights, and possible erroneous control conducted in some cases to significantly degrade the transmission characteristic.
JP-2001-217759-A in turn describes an adaptive antenna transmitting/receiving apparatus which controls transmission antenna weights based on an interference degree calculated by an interference degree calculating unit. However, this conventional adaptive antenna transmitting/receiving apparatus is similar in that it cannot calculate the interference degree itself if the reception line quality is degraded, resulting in the inability to control optimal transmission weights, and possible erroneous control conducted in some cases to significantly degrade the transmission characteristic.